Transmitting electrical energy through the air was one of the ideas Nicholas Tesla He came to patent more than a century ago. Now, researchers from universities in the US and Spain have managed to capture the energy contained in the signal emitted by WiFi devices and turn it into electricity. Using a nanomaterial of only three atoms in thickness, they designed an antenna that transforms electromagnetic waves into direct current. Although the power achieved does not exceed the range of the microvatios, the mechanical flexibility of the material and the omnipresence of the electromagnetic signals necessary to connect millions of computers and mobile phones to the internet bring the dream closer to an electronic system that is everywhere.
"All electromagnetic radiation contains energy," recalls the professor of electrical and computer engineering at the Massachusetts Institute of Technology (USA), Tomás Palacios. "It is not very different from sunlight, it only changes the frequency that, in Wi-Fi, is much lower," adds the head of the Materials and Advanced Semiconductor Devices Group at MIT and co-author of this research.
A special antenna (rectena) captures the wireless signal and converts its alternating current into continuous
What they have done has been to create a device that manages to capture the energy that the WiFi signal uses to transmit or capture data. This signal propagates in all directions even if the destination of the information is a single point. The rest is lost. To take advantage of it, they created a special antenna (rectena or rectifying antenna) that receives the radiation emitted by wireless devices, such as routers or wireless access points, but also any device with WiFi, such as laptops, televisions, mobiles, tablets … that use the same frequencies, that is, the bands of the 2.4 gigahertz (GHz) and 5.6 GHz.
The problem is that this environmental energy reaches the antenna as alternating current and must be rectified. "It's like a battery that changes its polarity continuously, to feed the electronic circuits we need a constant voltage," recalls Palacios. To achieve this conversion to direct current, and here is the great contribution of this research published in Nature, have designed a diode with a material that has physical, mechanical and electrical properties that neither graphene: molybdenum disulfide (MoS2).
Nikola Tesla has already patented a system to send electricity through the air more than a century ago
As the graphene, the MoS2 is a two-dimensional material. If the first has a thickness of one atom, the second has it of three. That gives them a flexibility that silicon or gallium arsenide will never have, on which current electronics and technology are based. Both are also easy and very cheap to produce. But, unlike MoS2, graphene is not a semiconductor, which limits its possibilities in the field of electronics.
In the rectena manufactured by Palacios' equipment, the energy captured by the antenna arrives as alternating current to one of the electrodes (anode, made of palladium) and leaves with constant polarity for the other electrode (cathode, gold). In between, the person in charge of performing the magic is molybdenum disulfide (MoS2) and does so at an ideal speed for the high frequencies used in WiFi signals. "It allows us to create a diode fast enough to rectify even in the 10 GHz band," says the professor of the microwave and radar group at the Polytechnic University of Madrid and co-author of the research, Jesús Grajal de la Fuente.
But for this engineer, as for Palacios, the key to his device is its enormous flexibility. "Faced with MoS2, silicon is a brick," he says. In addition to expensive and fragile, no matter how much miniaturization progresses, it will always be rigid. That is a feature that achieves its end only in two-dimensional materials and that would allow, for example, cover a wall or a whole building of sensors that would feed on the energy of the environment.
"What if we could develop electronic systems that we could deploy along a bridge or cover an entire highway or the walls of our offices and bring electronic intelligence to everything that surrounds us? Where would you get the energy for so much electronics?" , asks Palacios. There would be no batteries or enough plugs. Only by plugging them into the air, to wireless power, could you imagine something like that.
The creators of the system imagine a future where electronics is ubiquitous and feeds on environmental energy
In all this there is a limitation that turns out to be its great virtue. The power of the WiFi signal (and that of other wireless technologies, such as the 4th and 5th generation mobile communications) is by necessity very low. "WiFi needs around 100 microwatts, 100,000 times less power than required to light an LED bulb," recalls Palacios. So you can not load the laptop with a rectena of these.
"It is not enough, but it will be enough to feed all kinds of sensors, now the electronics are limited to macroscopic objects, the mobile phone, the computer, the car, in the future it will be ubiquitous, it will be in the clothes that we carry inside us. the buildings … and will take advantage of the energy of the environment, "says the Spanish professor at MIT.